Nigel Calder takes the pulse of science, as the author of Magic Universe and Einstein's Universe. He checks predictions of the past half-century, to see how they worked out. And his hand is on the brow of frenzied climatology, as a co-author of The Chilling Stars: A Cosmic View of Climate Change.

Comets from sister stars

Upating Comets

Did our comets come from sister stars?

How did the Solar System acquire its never-ending supply of comets to keep startling us? An explanation comes in today’s Science magazine, from Harold Levison and David Kaufmann of the Southwest Research Institute in Boulder, Colorado, working with Canadian and French colleagues.

The presumed source of supply is a very distant cloud of 100 billion or more comets, loosely bound to the Sun, called the Oort Cloud. The new report suggests that, in the tight cluster of stars in which the Sun was born, comets were scattered hither and yon in close encounters between stars, and many of our comets were captured from the Sun’s sisters.

In this extract from Comets I am at pains to stress that Jan Oort wasn’t the inventor of the distant comet cloud.

Ernst Öpik is an Estonian astronomer and musician who has recently been running the Armagh Observatory in Northern Ireland. For most of his long life he has adopted the role of cosmic garbage-sorter, concerning himself with the stray material of the Solar System. In 1932 he calculated that an invisible cloud of comets and meteors, surrounding the Sun at enormous distances, could survive throughout the long lifetime of the Solar System. In 1950 the doyen of Dutch astronomers, Jan Oort of Leiden, who is better known for classic work on the nature of galaxies, reworked Öpik’s idea. He emphasised a different aspect of it, namely that passing stars would cause a few of the objects to fall out of the cloud and into the heart of the Solar System, to become observable as ‘new’ comets.

Thus was the fabulous Öpik-Oort Cloud conceived, as the source of the comets. I abridge the name to the Öoo Cloud and defend this coinage on grounds of sight and sound. It looks like an untidy collection of roughly round objects of various sizes, and it is pronounced ‘Er, oh!’ – just what a neophyte comet lover is liable to utter when he is first told that there are many billions of the things out there.

The Öoo Cloud is fabulous in both senses of the word: amazing on the one hand, totally hypothetical on the other. As critics are quick to point out, it is by definition as invisible as a cloud of gnats a million miles away and there is no possibility of observing it directly – until perhaps the first starship leaves the Solar System and passes through the Cloud on the way out. But … I shall describe the Öoo Cloud on the supposition that it is real. The numerous comets are very slowly orbiting the Sun, coming no closer than the outermost planets and spending by far the longest phases of their existence at much greater distances, in the Cloud. Then you bring in the passing stars.

Edmond Halley himself first discovered 250 years ago that the stars which look so steady to the casual observer are, in reality, whizzing about at high speeds in relation to the Sun. For those 0f a nervous disposition who suspect, quite rightly, that a collision with a star would be even more disagreeable than a collision with a comet, let me say that the sky is a big place: during a ten-billion-year lifespan of the Solar System the closest that any star is likely to come is about five hundred times the distance from the Earth to the Sun. But the Öoo Cloud is a hundred times farther off than that, and as often as once in a million years a star will charge through a segment of it like a wild bull through a garden party, scattering the guests.

Perturbed by the star’s gravity, many millions of comets take flight, quitting the Solar System for ever. Others change their orbits drastically and yet remain in the Cloud. And ten million comets, for the sake of argument, are stopped in their tracks like bull-watchers paralysed with fear. As it was only their slow orbital motion that kept them at their distance from the Sun, these arrested comets can then do nothing but begin the long, long fall. But they will not be bunched: a number of star-encounters will occur during the time of several million years required to complete the fall, and any small residual speed (up, down or sideways) that a comet possesses at the start makes a huge difference to the time and direction of its arrival at the Sun.

Update to add to Comets

(it takes account of a much earlier suggestion on the same lines as today’s report)

Not properly explained was how the comets got into the Öoo Cloud in the first place. Arm-waving accounts said that comets forming in the Sun’s vicinity, as part of the early planet-building process, were booted out into the distant cloud by encounters with the massive outer planets. But calculations of what could be expected by that mechanism delivered at best only about one in ten of the 100 billion or more comets estimated to be present in the Cloud. So in 1990 Jia-Qing Zheng, Mauri Valtonen and Leena Valtaoja of the University of Turku in Finland suggested that the comets were born in a cluster of stars, together with the Sun, and from the swarm of comets each star acquired cometary companions.

The idea languished for twenty years until revived by Harold Levison and David Kaufmann of the Southwest Research Institute in Boulder, Colorado, working with Canadian and French colleagues. In their version in 2010, the comets of the Cloud originated in discs around the Sun and other stars, as potential ingredients of planets. They were then sent flying all over the place, not just by massive planets, but by close encounters with the far more massive stars born at the same time as the Sun – its sisters in a stellar nursery known to astronomers as an open cluster. The orphaned comets then found permanent homes if captured by Sun or another star. That would happen only if, by chance, the comets happened to be travelling in the same direction and with much the same speed as the star in question.

Many comets would be simply lost in space – a thought that led Zheng and his colleagues in Turku to suggest back in 1990 that “the present day density of comets in interstellar space has to be high, and … comets make a significant contribution to the overall mass density of the Galaxy.” So there you are – they’re not just the small change of cosmic creation.

A footnote: Ernst Öpik died in 1985.

In a computer simulation, to be followed from A to F, stars in a young cluster (red and yellow circles) are accompanied by comets from the planet-forming process (blue dots). The red star in the middle acquires comets from a close encounter with another red-coded star. Source: Levison et al., Science (see reference).

Jia-Qing Zheng, Mauri J. Valtonen, and Leena Valtaoja, “Capture of comets during the evolution of a star cluster and the origin of the Oort Cloud,” Celestial Mechanics and Dynamical Astronomy, Vol. 49, pp. 265-272, 1990.

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